Subsea wells are increasingly important to hydrocarbon recovery operations. Numerous land-based wells have been drilled, but the percentage of hydrocarbons recovered from land-based wells is steadily decreasing in some parts of the world. Jack-up rigs have been used offshore for decades to drill wells subsea to recover oil, but jack-up rigs are practically limited to drilling operations in relative shallow water of several hundred feet. As water depth increases other drilling rig options may be required to facilitate drilling and well completion operations. In addition to an increase in the number of off-shore wells being drilled, in more recent years an increasing number of wells are being drilled in deeper water and at increasing costs. Accordingly, drilling from offshore rigs, e.g., drilling ships, semi-submersibles, jack-ups, drilling barges or submersible rigs has significantly increased in recent years. The economics associated with drilling offshore remains, however, a primary reason why more wells are not drilled offshore. Particularly, in drilling exploratory wells where financial risk and commercial hydrocarbon uncertainty may severely impact the economics for drilling such wells, costs and may be more critical in determining whether any wells are drilled at all, and how many may be drilled.
The majority of offshore or marine drilling rigs utilize riser sections as the outermost tubular between the rig and the seafloor, with the riser sections typically being bolted, clamped, mechanically fixed by dog-type latch mechanisms or otherwise connected. Riser sections conventionally include hydraulic lines spaced outwardly of the assembled riser pipe for operating the blow out preventer (BOP) and subsea ram stack located above the mud line. During an emergency or in anticipation of adverse weather conditions, the subsea BOP may be closed and the rams hydraulically activated to seal off the well bore. Prior to closing the rams, the drill pipe may be threadably disconnected above or below the BOP stack utilizing a back off tool or back off method, or the drill pipe may be sheared by the shear ram assembly. In some applications, acoustically or electrically activated subsea accumulators have been used to replace the hydraulic lines which commonly are run along side the riser pipe. The subsea BOP stack assembly used during deep water drilling operations may contribute significantly to the cost of drilling a well and a substantial amount of expensive rig time may be expended running in and removing the riser pipe sections and related well control equipment.
The above disadvantages associated with drilling from floating drilling rigs have long been known. Accordingly, some drilling or operating companies may recommend “riser-less drilling” for certain deep water applications. A subsea pump may be provided to return the drilling fluid to the surface in a separate flow line. Riser-less drilling still has to contend with the high cost of the BOP stack and hydraulic operation of this equipment. Several wells have been successfully drilled from a floating drilling rig, while using a riser, wherein the BOP is placed on the drilling rig rather than subsea. To date, however, these wells practically are limited to geographic areas where and/or seasons when there is a reduced likelihood of adverse weather conditions which would require the floating drilling rig to relatively quickly disengage a portion of the riser, e.g., an upper riser from the lower riser. In these applications, however, elimination of the subsea BOP stack may result in significant cost savings when drilling a well. Further savings may be realized by using conventional threaded casing for a riser rather than flange-type riser pipe sections. Less area on the drilling vessel is required to store casing having the same nominal diameter as the riser pipe sections since conventional riser pipe sections include both flanges and hydraulic lines which are eliminated when using casing as the riser.
Typically, subsea BOP stacks are installed on the riser string. The BOP stack may be required to provide a subsea method of isolating a lower portion of the riser and well bore from the riser above the BOP stack. Stress in the riser typically includes the weight of the riser and the weight of the subsea BOP. Subsea BOP stacks may weigh in excess of 400,000 pounds. The weight of the BOP stack plus the weight of a riser sufficiently strong enough to deploy such stack and meet operational requirements necessitates that risers are inherently heavy pieces of equipment which may exert high levels of stress and strain on the drilling and on the riser sections. These effects may be even more pronounced in deep water applications. In deep water installations, installation of a typical riser system may require calm weather and well in excess of a week to install, and in excess of a week to retract. In addition to the subsea riser and BOP stack, electrical and hydraulic umbilical lines are typically deployed concurrently, to control and operate the BOP stack, choke and kill line valves, and hydraulic disconnects if present. Deployment and recovery of this equipment and the rig time involved all contribute significantly to well costs, as daily rental rates for semi-submersible drilling rigs may exceed $240,00 per day. Premature disconnection of a portion of the riser can likewise be expensive and time consuming, such as may be necessary in advance of hostile weather conditions, broken mooring chain or slipping mooring anchor.
If drill pipe is in a well bore and it becomes necessary to seal the interior of the well bore, pipe rams or shear rams in the BOP stack may be closed on the drill string to confine pressure and fluid within the well bore. In the event it becomes necessary to disconnect an upper portion of the drill pipe from a lower portion of the drill pipe, the drill pipe may be unthreaded at a tool joint, or cut with a chemical cutter or explosive charge. If pipe is stuck, the free point may be estimated by a free point calculation technique. Each of these disconnect methods requires time to determine free points, deploy appropriate tools on wire line, such as a “string shot,” a free-point tool, a chemical cutter or jet-shot explosive charge. Multiple attempts and re-calculations may be required. The process can be time consuming and frustrating and may still result disconnecting at an undesirable disconnect point. Reconnecting after disconnecting can be even more exasperating, time consuming and expensive, and even impossible.
Disadvantages of the prior art are overcome by the present invention. An improved method of drilling from a floating drilling rig is hereinafter disclosed. A subsea riser disconnect is provided for connecting and disconnecting a lower riser from an upper riser.